Continuous milling and devolatilizing apparatus



Nov. 12, 1968 F. P. KLOSEK ET AL 3,409,937

CONTINUOUS MILLING AND DEVOLATILIZING APPARATUS Original Filed June 28, 1963 INVENTORS Z FELIX e KLOSEK RICHARD E. NICOLSON SYDNEY P. SPENCE ATTORNEY United States Patent 3,409,937 CONTINUOUS MILLING AND DEVOLATILIZING APPARATUS Felix P. Klosek, Neshanic Station, Richard E. Nicolson,

Martinsville, and Sydney P. Spence, Westfield, N.J., assignors to Union Carbide Corporation, a corporation of New York Original application June 28, 1963, Ser. No. 291,362. Divided and this application Dec. 28, 1966, Ser. No. 630,151

2 Claims. (Cl. 18-2) ABSTRACT OF THE DISCLOSURE A twin screw milling and devolatizing apparatus is provided with means for injecting inert fluid e.g. steam into the devolatilizer at a point where the material being treated is at maximum temperature. The input conduit produces a fluid flow counter-current to the direction of the material being advanced by the screws so that volatile contaminants such as unreacted monomer, solvent and by-products can be readily swept off the advancing materials to a vapor outlet, leaving behind a product, e.g., polymer of high purity.

It is an object, therefore, of the present invention to provide apparatus wherein, with brief milling times, plastic materials are brought to very low levels of residual contamination.

It is another object to provide improved milling and devolatilizing apparatus.

Other objects will appear hereinafter.

This improved apparatus enables removal of contaminants from said plastic material simultaneously with the milling and conveying thereof in apparatus comprising in combination a pair of parallel rolls having intermeshing left and right-hand helical threads and beginning, middle and terminal portions with respect to the conveying of plastic material, said threads having a V-shape in vertical cross-section for at least a major portion of the length of the rolls and a shape square for any remaining portions of the rolls, a housing enclosing and supporting the rolls, said housing extending in part at least about the middle of the rolls to provide a vapor outlet and completely encircling the termnial portion and means for rotating the rolls in counter directions; a saddle positioned closely adjacent the V-shaped threads of the terminal portions of the rolls axially extending across at least one flight (axial length of a thread) thereof and closely conforming thereto to define therewith a space for permitting passage of ma-- terial corresponding to a zone of maximum heat for milled material; the improvement comprising a conduit leading from a source of inert fluid outside said housing to the space between the saddle and the terminal portions of the rolls whereby said fluid is contacted with the plastic material in said space and is passed out said vapor outlet.

In the drawings:

FIG. 1 is a view in elevation, partly in section, illustrating a preferred embodiment of the preferred apparatus;

FIG. 2 is a sectional view in elevation taken along line 2-2 of FIG. 1.

The advantages of the process and apparatus of the present invention are manifold. All plastic materials containing undesirable levels of volatile contaminants can be improved by the present invention. The term contaminants herein is used to refer to unwanted constituents of plastic materials and the term includes among other substances, solvents, decomposition products, unreacted reactants, additives both deliberate, such as modifiers, and accidental, such as moisture.

Specific instances of contaminants which are readily removable in accordance with the teachings of the present invention include these:

Removal of solvent, e.g. chlorinated solvents such as methylene chloride, from bisphenol-A polycarbonate. Heretofore the removal of this solvent, which can cause mold corrosion if not substantially completely removed from the plastic, has been attempted by conventional milling and stripping operations but these are believed to be responsible for a darkening of the milled product due to oxidation by incursive atmosphere. The devolatilization herein described removes solvents to acceptably low levels e.g. 300 parts per million based on the polycarbonate, by weight, and further provides the benefits of exclusion of atmosphere and cooling the plastic during milling thus inhibiting oxidative and heat degradation.

Removal of unreacted reactants, e.g. styrene from styrene homoand copolymerization products. Use of the present method enables increased milling rates with no increase in residual monomer content in the milled product.

Removal of color bodies and odoriferous fractions from plastic masses, e.g. polyethylene, such as the particle form polyethylene described in U.S.P. 2,825,721 to J. P. Hogen et a1. Injection of inert fluid (defined below) as herein described simultaneously deodorizes plastic masses and inhibits color body formations therein.

The term inert fluid herein refers to vapors, gases and liquids which are nonreactive toward plastic masses, thus the inert gases e.g., N and water and steam can be used.

Steam is the preferred fluid to be used in milling accomplished in accordance with this invention, but it is not necessary that steam as such, be introduced into the mill. Water can be injected. The temperature of the introduced fluid is not critical. It is desirably lower than the material it contacts to provide a cooling effect. In parallel demonstrations of stripping polystyrene of monomer using the mill herein described operated at 53 revolutions per minute, a 270 C. jacket temperature and a 77% solids feed stream (ca. 13 pounds per hour) results obtained were (a) without steam, 15% monomer in the product (b) with 20 pounds per hour of steam (1.5/1 H O/polymer ratio; velocity=30 ft./sec.), 1.8% monomer in the product and (c) with 1.1 pounds per hour of water 0.08/1 H o/polymer ratio; velocity 1 ft./sec) 4.6% monomer in the product. All percentages are by weight.

Refering now to the drawings in detail, there is shown an apparatus comprising two parallel rolls 10, 12 of steel or other suitable material with intermeshing helical V-type threads 14, 16 completely enclosed by housing 18. These rolls lie with their axes in a horizontal or vertical plane. One thread 14 is left-handed or counterclockwise (viewed from the discharge end and the other thread 16 is righthanded (clockwise); and the rolls are rotated in opposite directions (shown by arrows) so that the intermeshing surface areas move together upwardly to provide the bite.

Material is fed to the bite provided by the radial clearance between the rolls in the zone of intermesh through inlet port 20 at a controlled rate. The threads on each roll can be divided into two sections, 22, 24 each of a different type; the major section 22 consists of V-threads for milling and extends about three-fourths the length and includes the beginning and middle portions of the rolls, while the terminal portion 24 has square threads for pumping. Both rolls are supported in the cast housing for rotation in a saddle shaped dual base 26, of a diameter to closely accommodate the rolls; but above the thread sections 22 a portion of the saddle shaped dual base in the block is absent to open into vapor space 28. Bearing 30 in wall 32 seals the rolls at that end; bearing 34 in wall 36 seals the rolls at the discharge end.

The roll shafts extend through the housing 18 to the drive means, timing gears, reduction gears and motors as is well known in the art.

Housing 18 has a discharge port 38 opening in the base of the block 29 through which the milled, devolatilized material is extruded or forced by the positive pumping action of the square thread sections 24 of the rolls.

Novelly with the present invention, there is provided over the terminal portion of the rolls 10, 12, a ported sadtile 40 closely spaced above the threads and extending axially thereover a distance of at least one flight and preferably extending several flights where there are few leads per flight. The edge 44 of the block 40 can be tapered to minimize hang-up of material passing thereby. The thread crest to saddle distance should be just suflicient to enable passage of material. This insures intimate contact between steam and material at the zone of maximum material heat, due to the degree of milling material this zone has received. A typical construction will be a mill having four inch diameter rolls with a spacing of 0.030 inch between thread crest 42 and block 40. Generally spacing will range from 0.01 to 0.2 inch. Constant or varying spacings may be employed. Provded in the saddle 40 is at least one conduit 48 having inlet 48 leading from an inert fluid source outside the mill (not shown) and outlet 50 from which inert fluid is discharged into the mass of material being milled. Provision of a plurality of outlets 50 circumferentially spaced about the rolls is preferred. The velocity at which inert fluid is moved over the surfaces of the material in the zone of maximum material heat between the saddle 40 and the threads 42 for best results should be at least one foot per second and can be 200 feet per second and higher. The fluid carrying away the stagnant vapor phase formed on the milled material during mill operation enters the vapor space 28 and drawn out vapor outlet 54. Steam to product ratios are generally in the range of 0.01:1 to 2:1 and preferably are in the range of 0.5 to 1.5, especially 1: 1. All ratios are by weight.

Example Normal Goncur- Counter Atmos- Vacuum rent current; porlc Steam Steam P.p.m. Cl 3,400 273 33!) 34 Percent. yellow 22 12 12 Percent white 83 87 87 What is claimed is:

1. In an apparatus for devolatizing plastic material having twin screws rotatably mounted in close proximity, an input port at one end of said screws, a discharge rt at the other end thereof and a vapor port proximate the input end, with a saddle plate positioned in close clearance about the screws proximate the discharge end thereof, which plate defines with the screws a zone of maximum material heat for the treated material, the improvement comprising, a fluid input conduit terminating between said saddle plate and said screws for producing a fluid sweep counter-current to material being advanced by the screws so that any stagnant vapor contaminants on the advancng materials can be swept from said materials and carried out said vapor port.

2. An apparatus claimed in claim 1 wherein there is provide a plurality of outlets in said saddle into the zone of maximum material heat circumferentially spaced about the rolls.

References Cited UNITED STATES PATENTS 2,434,707 1/1948 Marshall 182 XR 2,615,199 10/1952 Fuller 182 XR 2,836,851 6/1958 Holt.

3,040,005 6/1962 Bernhardt et a1. 1812 XR 3,082,816 3/1963 Skidrnore l812 XR 3,147,514 9/1964 Reilly 182 3,156,009 11/1964 Alsys 18--2 XR 3,257,173 6/1966 Parnell 1812 XR WILLIAM J. STEPHENSON, Primary Examiner. 

